An idiom is an expression formed with a group of words that presents a figurative meaning than literal meaning.
'To become a vegetable' is an idiom which means that someone has become <u>physically disabled</u> or has ceased from doing any physical tasks.
In the given sentence, the correct word to be used is vegetable. It is because the person (she) is rendered physically disabled as she is in a coma on a life support machine.
